Old style figures and small caps

I'm getting in a bit of a muddle with my old style figures, and wonder if somebody can clear this up for me?

I'm setting a long text in Caslon Pro, using the Opentype version with its Opentype old style figures throughout the text. I noticed that words and acronyms involving both numbers and letters looked very odd, so I decided that I probably should be using Opentype small caps for acronyms. Is that the typographically correct approach? If so...

- What do I do with acronyms in italics (e.g. a magazine whose name is an acronym)? There are no italic small caps in the font that I can see, so do I just create an inconsistency, or should I fake it using Indesign's Small Caps (not the Opentype small caps)?
- Are there some clear rules on when and when not to use small caps, for example, I'm unsure about the X in X-rated?

Proper small caps usage demand a set of small caps figures. Some recent fonts already have this (Greta Pro, Comenia Pro, etc.). Lining figures are almost unusable with small caps a-z and the old style ones seem odd, somewhat misaligned. Anyway, if you don't have small caps figures, the old style is preferable.

Proper small caps also demand complete style/weight variation. I never understood these fonts with SC in regular styles and without SC in italics. This makes no sense.

I think the ideal solution is to have SC a bit higher than x-height, including figures and punctuation, and petite caps set exactly at the x-height. Vesper Pro adopts this. So you can handle mixed acronyms (like PostgreSQL) and have numbers perfeclty fitting the small caps height.

I manage mostly technical and legal texts with lots of acronyms and numbers. The sub optimal result I get with SC and oldstyle figures in almost all fonts was the main reason I decided to develop my own font.

I'm not sure if there is an alternative to Caslon Pro with complete SC features. Maybe the beautyful Calluna.

Common practice is to use full caps for three capitals in a row or fewer, and small caps for four or more and A.M. and P.M. You can use small caps for educational degrees too, as long as you use periods.

If you decide, just because you have so many of them, that your own style should put three-letter acronyms in small caps, that will probably fly, but not two letters or one. So no, not for X-rated.

In my opinion, if you absolutely need to fake italic small caps, it's better to slant real small caps than to fake small caps with real italics.

I’d consider Adobe Caslon Pro a book face but also a workhorse, so the lacking Italic Small Caps is a bit unexpected even though you rarely need them in book work. More complex work could definitely use both Italic SC and Petite Caps.

I had to fake some small caps (just a couple of words) in a recent project. I decide to scale down the italic capitals, added a thin stroke to the shapes to match the weight and also nudged them ever so slightly up from the baseline (the same distance as the added stroke). I also streched them by about 110% and added a bit of tracking.

Which way to go depends how much the italics differ from the roman and on the size you are using.

IMO, speaking as a graphic designer, if you have two sets of SC the smallest should be slightly taller than the x-height and the largest just below the capitals (useful in capital heavy languages like German, and for genetive case with names in all caps). That’s up to the type designer, anyways.

Thanks for your interesting replies.
In the end I chickened out and ditched the old style figures completely - a decision made reluctantly but I couldn't quite bring myself to fake it on the italics and others potentially updating the file in years to come won't necessarily understand what's been done and where. I'll definitely look out for italic small caps in future, but this particular project was too far down the line for a change of typeface.
Out of interest, what's the distinction between small caps and petite caps (@freiberger)? @frode frank you also mentioned having two sets of small caps, what are they exactly?
Thanks!